This invention relates to radio frequency transponders.
Passive radio frequency transponders derive energy from an interrogation signal which is used to energise them, as opposed to active transponders which include an energy source such as a battery. This makes passive transponders relatively cheap to manufacture. However, the lack of a built-in energy source and the need to extract energy from the interrogation signal limits the effective operating range of such transponders for a given intensity of the interrogation energy field. Conversely, where significant operating ranges are called for, the amount of power which must be radiated by an antenna generating the energy field increases, to the point where it may infringe certain safety regulations or other legislation.
It is an object of the invention to provide a radio frequency transponder arrangement which can operate at increased ranges or with lower power energising fields than prior transponders.
According to the invention there is provided a method of matching an electronic circuit of a radio frequency transponder having a complex input impedance to an antenna, the method comprising:
measuring the complex input impedance of the electronic circuit;
calculating a first matching element value to transform the complex input impedance to an intermediate impedance value;
calculating a second matching element value to transform the intermediate impedance value to a real value corresponding to the antenna impedance; and
constructing a matching network with inductive and capacitive matching elements corresponding to the calculated first and second matching element values.
The complex input impedance may have a negative reactance component, with the first matching element value being inductive and the second matching element value being capacitive.
The method may include constructing a matching network comprising a capacitor in parallel with the antenna and an inductor in series with the antenna and the electronic circuit.
Preferably, the method includes forming at least one of the inductive and capacitive elements of the matching network integrally with the antenna.
Further according to the invention a radio frequency transponder assembly comprises:
an antenna;
a transponder circuit including a power supply circuit arranged to be fed with energy received by the antenna, a logic circuit, and a modulator circuit for generating data signals for transmission by the antenna; and
at least two impedance matching elements, at least one of the impedance matching elements being formed integrally with the antenna.
The transponder circuit may have a complex input impedance, with the impedance matching elements comprising a capacitor and an inductor and together effectively defining an impedance matching circuit between the antenna and the transponder circuit.
Preferably, the complex input impedance of the transponder circuit has a negative reactance component and the antenna is a dipole or folded dipole.
The matching circuit may comprise a capacitor connected in parallel with theantenna and an inductor connected in series between the antenna and the transponder circuit.
Preferably the capacitor is connected between first and second terminals of the antenna in parallel with input terminals of the transponder circuit.
The inductor is preferably connected in series with a first terminal of the antenna and an input terminal of the transponder circuit.
Typically, the antenna has at least one element comprising a first length ofconductive material, and the inductor comprises a second length of conductive material connected between the antenna and the transponder circuit, the inductance thereof being determined by said second length.
For example, said at least one antenna element and said inductor may comprise a conductive metallic foil.
In one version of the invention, the antenna has at least two elements and the capacitor is formed by overlapping adjacent elements of the antenna separated by a dielectric layer.
Said at least one antenna element and said inductor may comprise a metallic wire.
Alternatively, the antenna may have at least two elements, with the capacitor being formed from a length of wire connected to one element of the antenna and wound around another element thereof.
A preferred radio frequency transponder of the invention has an operating frequency in the range of 440 to 930 MHz and an operating range of at least 1 m for an effective radiated power of a reader energising the transponder of no more than 500 mW.
To produce low cost transponders, designers need a simple antenna system to which is connected a transponder integrated circuit containing all the electronic components. Such components would typical comprise the rectifying diodes, storage capacitors, modulator, an internal low frequency oscillator, memory storage and logic circuitry. Typical designs use voltage doubling rectifying circuitry to extract the operating power, and backscatter modulation to relay the transponder""s data to the reader This type of design results in only the rectifying circuitry and the modulator experiencing radio frequency excitation,while all other parts operate at a relatively low frequency generated by the oscillator.
Different countries regulate the use of the radio spectrum within their zones according to their regional plans. These plans often limit the strength of energising fields that may be used in their region, and the present invention is intended to allow transponders to operate at a reasonable range from the source of the reader""s energising field in those regions where very low energising field powers are permitted.
In order to maximise the energy transfer from the energising field via the antenna to the rectifying circuitry, ideally a matching network is needed between the RF electronic components (rectifiers, modulators, and energy storage), and the antenna. The matching network should convert the actual complex input impedance of the transponder""s electronic components to the conjugate impedance of the antenna. In conventional designs this might be achieved using transmission lines, or combinations of inductors and capacitors.Due to the large values needed for the inductors and the capacitors and the poor manufacturing tolerances achieved with integrated circuit manufacture, as well as the effects of temperature on the nominal values of such components, it is not commercially viable to include-these components inside the electronicintegrated circuit.
Without a matching network a typical electronic circuit might exhibit an input impedance of 8.86xe2x88x92j29.67 ohms while a dipole has an impedance ofapproximately 72xe2x88x92j0. At the junction of the antenna and the electronic circuit the imbalance of the impedances would cause 70% of the incoming power to be immediately reflected and not converted to useful energy by the transponder. To compensate the energising field would need to be increased by more than 3.3 times to deliver the same energy to the rectifying diodes compared to that of a properly matched situation.
The present invention provides apparatus that provides the necessary matching cost effectively, and a method that can be used to compute the correct values of the matching network.